The present invention relates to a fixing device that fixes a toner image on a recording material in a nip portion that is formed by a fixing member and a pressure-applying member.
In an image forming apparatus of an electrophotographic type such as a copying machine, a printer, a facsimile machine and a multifunctional peripheral equipped with various functions of the aforesaid items, a latent image corresponding to a document is formed, then, the latent image is visualized by receiving toner particles, and the visualized toner image is transferred onto a recording sheet and after that, the toner image transferred onto the recording sheet is fixed to be ejected.
As a fixing device that fixes a toner image in the aforesaid way, there is available a fixing device of a heat roller fixing type wherein a recording sheet onto which a toner image has been transferred is heated and is given a pressure in a nip portion that is formed by a fixing roller having therein a built-in halogen heater and by a pressure-applying roller that applies pressure to the fixing roller, while being interposed and conveyed, and the fixing device of this type is widely used because of its simple structure.
Further, there is available a fixing device of a belt-fixing type wherein a fixing belt in an endless form is trained about a heating roller having therein a built-in halogen heater and about a fixing roller, and a pressure-applying roller that applies a pressure to the fixing roller through the fixing belt is provided, and a recording sheet onto which a toner image has been transferred is heated and pressed while being interposed and conveyed in a nip portion that is formed by the fixing belt and the pressure-applying roller, and the fixing device of this type has an advantage that warming-up time is short because thermal capacity of the fixing belt is small, resulting in energy conservation.
In this case, since toner of the toner image on the recording sheet is heated when it passes through the nip portion, the toner has adhesion, and the recording sheet that has passed through the nip portion sticks to surfaces of the fixing roller and the fixing belt to wind itself around the fixing roller and the fixing belt, without being separated to cause a fear of occurrence of a jam. With respect to a recording sheet, in particular, when a sheet (thin paper) having small basis weight that is a coated paper for printing having small basis weight is used, releasing efficiency is more declined.
On the other hand, when a fixing roller is made to be greater for securing a sufficiently long nip width, to meet speeding up of an image forming apparatus, a curvature on the roller at a way out of a fixing nip is also made to be smaller, which declines the releasing efficiency.
For the purpose of obtaining easier separation of a recording sheet from the fixing member, there are taken various measures including using heat-resistance resins having high releasability for an outer layers of a fixing member, coating release agents such as silicon oil and making toner to contain wax that is dissolved by heating and functions as release agent. However, primary factors to lower releasing efficiency such as image forming on the aforesaid coated paper and adhesive force caused by an increase of toner amount caused by overlapping of plural colors which form color images, have been increased, which makes a separation auxiliary device to be indispensable.
As a separation device, there is available a method wherein a releasing claw that is coated with fluorocarbon resin having excellent releasability is provided on the sheet-ejection side for the recording sheet for the nip portion, and a tip portion of the releasing claw is caused to touch an outer surface of the fixing roller or the fixing belt, so that the recording sheet may be released from the fixing roller.
However, the aforesaid method has a problem that scratches are caused on an outer layer formed by fluorocarbon resin that covers a surface of the fixing roller, because the tip portion of the releasing claw is in contact with an outer layer of a surface of fixing roller, and the scratches are transferred also onto an image. In the case of color images, a surface of the fixing roller, in particular, these scratches are in a tendency to appear remarkably, because glossy images are required.
To deal with the problem of this kind, there has been developed a technology to cause a recording sheet to release from the fixing roller by blowing air against the way out side of the nip portion.
As an example of the foregoing, there is known a sheet releasing device that is corresponded to a high speed copying machine by storing compressed air generated by a compressor in two air boxes, and by jetting the compressed air through reciprocal opening and closing of two electromagnetic valves connected to the air boxes (see Unexamined Japanese Patent Application Publication No. S60-256180).
Further, there is known a sheet releasing device that can release a sheet without fail through a method wherein plural air bag manifolds that blow air in the circumferential direction of a soft roller are arranged, and when the first air blowing fails to release a sheet, the second air blowing releases the sheet (see Unexamined Japanese Patent Application Publication No. S61-62087).
There is further known a fixing device wherein a separating claw (a releasing claw) is provided, and air sent by a fan is blown against a nip portion (see Unexamined Japanese Utility model Application Publication No. S63-140571).
Further, there is known a releasing device wherein a releasing auxiliary plate is arranged to be close to a nip portion, and compressed air in a pulse form is discharged through a clearance between a fixing roller and the releasing auxiliary plate (see Unexamined Japanese Patent Application Publication No. 2004-212954).
In addition to the foregoing, there is known a fixing device wherein compressed air generated by a compressor is discharged through two electromagnetic valves, and when a leading edge of a recording sheet passes through a nip portion, high pressure compressed air is jetted, and after that, low pressure compressed air is jetted (see Unexamined Japanese Patent Application Publication No. 2007-86132).
Releasing of a recording sheet from a fixing roller by blowing air so that the fixed recording sheet may not stick to the fixing roller, is based on an area of the portion that receives the air blown against. When a toner image does not exist on the vicinity of the leading edge and when an area where no adhesive force is generated is broad, the leading edge of the sheet is released by “stiffness” and the empty weight of the sheet itself, and the area thus released receives air, which makes it possible to give a large releasing force. However, if images are in existence up to the vicinity of the leading edge of the sheet, a clearance formed between the leading edge of the sheet and an outer circumference of a fixing roller is extremely small, because the sheet follows in the tangential direction on an outer circumference of the fixing roller. For example, when the outer diameter of the fixing roller is 90 mm and a margin on the leading edge of the sheet is 3 mm, its clearance is only 0.1 mm. To make the leading edge of the sheet to be lifted by blowing air into the clearance, it is necessary to blow air having high air speed against the nip portion, in other words, to blow high pressure air against the nip portion, and for this purpose, compressed air generated to be at high pressure by a compressor is desirable.
A phenomenon that the recording sheet that has passed through the nip portion is not released and sticks to surfaces of the fixing roller and the fixing belt to wind itself around the surfaces of them takes place on the leading edge portion of the recording sheet and also takes place on other portions after the leading edge portion. Further, even when the leading edge of the recording sheet is drawn out by a sheet-ejection roller, uneven separation is caused on the twinning portion. Therefore, in the case of a structure to release a recording sheet by a fixing roller or the like by blowing air against the way out side of the nip portion, it is necessary to blow air continuously.
When blowing compressed air continuously as stated above, compressed air that is nearly equal to 0.01 m3/s is needed. For generating compressed air in this air flow amount, a high-power compressor of 5 to 10 kw is needed, because a lot of energy is required for generating compressed air, and there is a fear that a large-sized apparatus in a size of about 1 m3, composed of a compressor and an air tank is not avoidable.
On the other hand, in the conventional documents, it is not possible to find out a structure that satisfies various related functions, while preventing a large size of the apparatus of this kind.
Namely, in Unexamined Japanese Patent Application Publication No. S60-256180, there is a fear that a large-sized apparatus composed of a high-power compressor as stated above is not avoidable.
In Unexamined Japanese Patent Application Publication No. S61-62087, there is a fear that a large-sized apparatus composed of a high-power compressor as stated above is not avoidable, because a plurality of air bag manifolds are arranged.
In Unexamined Japanese Utility model Application Publication No. S63-140571, although air sent by a fan is used, it is impossible to obtain high pressure air in this case, and combined use together with a releasing claw cannot be avoided. Therefore, the problem that scratches are caused on an outer layer of the fixing roller by the releasing claw is not solved.
In Unexamined Japanese Patent Application Publication No. 2004-212954, there is a fear that an image on a recording sheet is damaged by scratches, because of the construction wherein a recording sheet is separated by a sharp releasing auxiliary plate, after the leading edge of the recording sheet is released by compressed air.
In Unexamined Japanese Patent Application Publication No. 2007-86132, compressed air is made to be at low pressure after the leading edge of a recording sheet is released by compressed air. However, for maintaining releasing efficiency, sufficient air flow amount is needed, and when obtaining this air flow amount, there is a fear that a large-sized apparatus composed of a high-power compressor as stated above is not avoidable.
The present invention has been accomplished in view of the aforesaid problems, and its object is to suggest a fixing device with a structure to release the leading edge of a recording material by compressed air wherein a high-power compressor is not needed and the device is not enlarged and an image forming apparatus equipped with the aforesaid fixing device.